The invention relates to a label with a diffractive bar code as set forth in the classifying portion of claim 1 and a reading arrangement for recognizing information on such labels as set forth in the classifying portion of claim 13.
Such labels are used for identifying articles, identity cards or passes or stock bonds and bear numerical information relating to the article, the identity card or the bond. The bar code of such labels is read off by optical means and is distinguished by good machine readability of the information contained in the bar code.
Various kinds of bar code are known, such as for example in accordance with MIL-STD-1189 or in accordance with the “European Article Numbering Code”, in which an item of information is contained in the arrangement of bar elements and intermediate spaces, of various widths. The bar elements are applied in a color contrasting with the intermediate spaces, to a carrier, usually paper, by means of a simple printing process. Reading apparatuses which can read off such bar codes are commercially available.
In accordance with U.S. Pat. No. 5,900,954 the level of safeguard of the bar code against forgery can be increased by the bar code being printed onto a carrier with a hologram. The bar code extends entirely or at least partially over the hologram.
EP 0 366 858 A1 describes various configurations of diffractive bar codes which, instead of printed bar elements, have surface elements with diffraction gratings. In comparison with the bar codes produced by a printing process the diffractive bar codes have a high level of safeguard against forgery. It will be noted however that the advantage of the high anti-forgery safeguard is achieved at the expense of a tolerance, which is low in comparison with the bar codes produced by a printing process, in regard to orientation of the diffractive bar code with respect to the reading beam of the reading arrangement, and a limitation in terms of the distance between the reading apparatus and the label, to a few centimeters. In addition the bar code which is produced individually by a printing process, with individual information, is extremely inexpensive while the diffractive bar codes can rationally be produced at viable costs, only in large quantities with identical information.
Surfaces arranged in a mosaic-like configuration, with microscopically fine diffraction structures which are embossed into plastic material, are known for example from EP-0 105 099 B1 and EP 375 833 B1. Design configurations of security labels with structures having an optical diffraction effect and the materials which can be used for that purpose are summarized in U.S. Pat. No. 4,856,857.
DE-OS No 1 957 475 and CH 653 782 disclose a further family of microscopically fine relief structures having an optical diffraction effect, under the name kinoform. It is only when the kinoform is illuminated with substantially coherent light that the light is deflected by the kinoform asymmetrically into a single spatial angle which is predetermined by the relief structure of the kinoform.